Trying to understand my plumbing system

As a DIY newcomer I have been reading up about plumbing systems with a view to trying to understand my own plumbing system, but I don't seem to be quite able to get to the bottom of it....
My 2-storey house has no loft, storage tanks, hot water cylinders or airing cupboards of any kind. All the hardware is in the garage, which is separated from the house by a 1 metre-wide path. In the garage there is a Potterton Netaheat Profile boiler, and a Megaflo unvented HW storage unit. So I have an unvented mains-pressure system, which explains why all the H and C taps throughout the house are so powerful - nice!
At the top of the garage rafters there is a small, well-lagged water tank. The top and bottom of the tank are connected by a pipe loop, and a branch from this loop feeds the boiler's "flow pipe", which in through another branch feeds the HW tank (this bit seems clear). Another pipe, this one well-lagged, from about the middle of the tank seems to disappear under the ground towards the house.
Things I would like to know:
- I don't quite understand why I need the tank - purely because the boiler isn't designed to be fed mains-pressure water? All the "unvented systems" diagrams I have read up on omit tanks of any description. - Why is the tank so well-lagged? Doesn't all HW go to the Megaflo? - Clearly the Megaflo feeds the H taps and showers in the house - presumably it also feeds the radiators? Or might that be the job of the lagged pipe from the lagged tank in the garage roof that seems to go under the ground towards the house?
Sorry if this is so basic!
Thanks for any advice.
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Richard Clay wrote:

Clearly you have not seen diagrams for the correct layout of an unvented HWC as opposed to boilers with unvented primaries.

Against frost.
Doesn't all HW go to the Megaflo? No presumably the boiler also heats the radiators.

No the boilr will do that.
Or might that be the job of the lagged pipe

Almost certianly.
OK High tank in garage is the header tank to the boiler and rads. It will be just(?) higher than than the highest rads in the house?
IIRC the connection to the Megaflow that is about 1/3-1/2 the way down will be the releif valve.
There are five holes on the Megaflow 3 connect to the HW in the cylinder, the other 2 are the coil connections to heat the water in the cylinder and connect to the boiler. The top is the HW outlet, the bottom one is the CW inlet and the other the releif outlet.
HTH
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Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
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boiler
systems"
Thanks for your advice. Could you just clarify the bit above please? What exactly is the difference between the 2 setups you mention? What's a "primary" in this context? (I assume "HWC" is "HW container"?)
Thanks again Richard
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Richard Clay wrote:

The HWC (HW cylinder) is unvented, that is there is no way for the water to leave the cylinder except via the taps (or when there is a problem via the releif valve). This contrasts with the normal HWC often found in a first floor airing cupboard where the top connection leads to a vent pipe which goes up into the loft higher than the cold water tank. Were the water in a vented cylinder to boil (which can happen if an immersion heater fails) then water/steam escapes via the vent pipe.
The water in the boiler and rads is sperate from the HW used a the hot taps. This water and its circuit is known as the primary. This water comes form a small open header tank (vented) or is sealed (unvented) see the FAQ below. You have a vented primary circuit fed from the small well lagged tank.

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Ed Sirett - Property maintainer and registered gas fitter.
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